The present invention relates to the specific field of turbomachines and is more particularly concerned with a disposition of the aerodynamic flow duct of a compressor stator of these turbomachines.
In a turbojet or a turboprop (referred to in the subsequent description as a turbomachine), it is known that the use of variable-pitch airfoils improves the general efficiency of the engine. Such is the case for the variable-pitch vanes of the compressor. However, it is also known that the clearance existing between the ends of these vanes and the walls of the flow duct has an adverse influence on this efficiency, a large clearance being a significant source of losses. Indeed, with variable-pitch vanes, the losses increase in proportion to the rotation of these vanes. French Patent Application No. 2 443 577 dealt with this problem and in order to solve it proposed a specific disposition of the inner aerodynamic flow duct. Unfortunately, this disposition suffers from two major drawbacks. Firstly, it necessitates an inclination of the axis of pivoting of the pivot of the vane at the level of the outer duct, and subsequently, the machining of the inner duct over an entire circumference, by hampering the flow between the vanes (through the creation of turbulence), introduces unnecessary losses of efficiency.
A subject of the present invention is a turbomachine compressor stator, the disposition of whose flow duct makes it possible to obtain an appreciable improvement in both efficiency and surge margin (flow separation limit) as compared with the prior art devices. An aim of the invention is also to propose a compressor stator allowing optimization of this efficiency over the entire range of operation of the vanes, from their open angular position to their closed angular position.
These aims are achieved by a turbomachine compressor stator comprising a plurality of variable-pitch vanes, each vane mounted in rotation, about an axis of pivoting, in a wall forming a surface of revolution about a longitudinal central axis of rotation of the turbomachine, being inclined by a specified angle xcex1 with respect to a plane perpendicular to the said longitudinal central axis, characterized in that the said wall is machined locally opposite an end part of each of the vanes so as to form a plurality of spherical portions or pockets whose respective centres are positioned on the said axes of pivoting and whose radii are determined so as to ensure, over the entire range of operation of the vanes, a predetermined minimum clearance between the said end part of the vane and the said spherical portion of opposite machined wall whilst limiting the depth of machining of this spherical portion.
The said end part of the vane is machined with a shape complementary to that of the said spherical pocket so as to ensure a constant clearance between the said wall and the said end part of the vane.
With this particular structure, a constant and minimum clearance can easily be obtained over the entire range of operation of the vanes.
According to a preferred embodiment, the said wall is the outer wall, or casing, of the compressor stator, the said end part of the vane being the upper end of this vane. Preferably, the said upper end part is a part of the vane overhanging at the rear, also known as the heel, connected with a trailing edge of this vane. Advantageously, the compressor stator furthermore comprises an unmachined part between two adjacent spherical pockets.
The invention also relates to any compressor of a turbomachine comprising a compressor stator such as described above.